A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
A lithographic apparatus includes in general multiple actuators, for instance to position a patterning device (e.g. a mask) and/or substrate support structure. These actuators may use the interaction between magnetic fields, e.g. produced by permanent magnets, and current-carrying conductors, in particular in the form of a coil, to apply forces to objects. Such actuators may be of a rotary, linear, or planar type and may have a moving coil or moving magnet configuration.
In the abovementioned actuators, the current-carrying conductors, i.e. the coil, may exhibit some kind of electrical resistance, so that a current running through the coil will generate a certain amount of heat. This effect gets even worse when the generated heat also causes an increase in electrical resistance of the coil, thereby further increasing the amount of generated heat, which in turn causes a further increase in electrical resistance of the coil, etc. Eventually this may lead to failure of the coils. The generated heat may also negatively influence the position accuracy of an object, e.g. a mask or substrate support structure, positioned by the actuator. Therefore, the coil is cooled, preferably to a constant temperature.
Cooling can be provided by a cooling body. This cooling body is preferably attached to an object or frame to dump the heat from the coil. A layer of well-known potting material is arranged between the cooling body and the coil to electrically isolate the coil from the cooling body, to transfer forces from the coil to the cooling body, and to transfer heat from the coil to the cooling body.
Thermal expansion differences may exist between the coil and the cooling body due to temperature differences between the coil and the cooling body and/or differences in thermal expansion properties of respective materials. Due to the difference in thermal expansion, stresses occur in the layer of potting material between the coil and cooling body. The maximum allowable stress in the layer of potting material limits the allowable heat generation. As the heat is generated by the current running through the coil, and the magnitude of the current determines the force the actuator applies to an object, the maximum allowable stress in the layer of potting material limits the maximum performance of the actuator in terms of maximum force.